1. Field of the Invention and Related Art Statement
The present invention relates to an apparatus for optically recording information on and/or from an opto-magnetic record medium including a thin film of an opto-magnetic material.
FIG. 1 shows a pick-up device of a known apparatus for optically recording information on an opto-magnetic record medium. Hereinafter, such an apparatus is called an opto-magnetic recording apparatus for the sake of simplicity. The apparatus comprises an objective lens 20 which is secured to a supporting frame 22 at its center, and a focusing coil 21 is wound around the frame. The focusing coil 21 is inserted into air gaps formed in a magnetic circuit which is constituted by yokes 23a, 23b and permanent magnets 24a, 24b secured to the yokes. The supporting frame 22 is connected to an intermediate member 22a by means of four leaf springs 25a, 25b and the intermediate member 22a is coupled with a fixing member 27 by means of four leaf springs 26a, 26b. To the fixing member 27 is also secured the yoke 23a. The fixing member 27 and the yoke 23b are secured to a base member 28. On opposite side surfaces of the supporting frame 22 there are secured flat tracking coils. In FIG. 1, these flat tracking coils are not shown. By supplying a current corresponding to a focusing error to the focusing coil 21, the objective lens 20 is moved in a Z axis direction which is parallel to the optical axis of the objective lens, so that a light beam which is projected by the objective lens toward a record medium (not shown) can be correctly focused on the opto-magnetic layer in the record medium. Further, when currents corresponding to a tracking error are supplied to the flat tracking coils, the objective lens 20 is moved in a tracking direction which is perpendicular to the Z axis as well as to the plane of the drawing of FIG. 1. In this manner, the light spot can correctly follow the information track in the record medium.
FIG. 2 is a schematic view illustrating the construction of the magnetic circuit of the known pick-up device shown in FIG. 1. In the known apparatus, the magnetic circuit is opened towards the side of the objective lens 20 and thus there is generated the leakage of magnetic field. In FIG. 2, the direction of magnetic flux is shown by arrows.
The leakage of magnetic field forms a magnetic field which is substantially parallel to the optical axis of the objective lens 20 and this leakage of magnetic field is superimposed upon the bias magnetic field which is applied to the opto-magnetic record medium for effecting the recording or erasing. Therefore, the leakage of magnetic field disturbs the bias magnetic field and the recording or erasing operation could not be performed accurately.
The recording operation for the opto-magnetic record medium is explained much more in detail hereinafter.
FIG. 3 is a schematic view of the pick-up device. The opto-magnetic record medium is generally formed to have the multi layer construction consisting of substrate 29, protection layer 30 and record layer 31 made of magnetic material. Above the opto-magnetic record medium there is arranged a permanent magnet or coil 32 for producing the bias magnetic field. In FIG. 3, the permanent magnet 32 is arranged on one side of the record medium opposite to the pick-up device. A light beam emitted by a light source is projected by the objective lens 20 onto a desired position of the magnetic layer 31 of record medium, while the objective lens is positioned at a given position by means of the driving circuit including the magnets, yokes and focusing and tracking coils.
FIG. 4 is an enlarged view showing the manner of recording or erasing the information. A part of the record layer 31 which is irradiated by the light beam is heated locally. The magnetic material of record layer 31 is heated above its Curie point, its magnetization is decreased abruptly. Next, the record layer 31 is cooled while the record layer is subjected to the bias magnetic field generated by the magnetic flux producing device 32 as shown by broken arrows. Then a portion of the record layer which has been heated by the light beam is magnetized in the vertical direction which is the same as the direction of the external bias magnetic field as illustrated by the thick solid arrow. In this manner, the previously recorded information can be erased or new information can be recorded. The direction of the magnetization is perpendicular to the plane of the record layer 31.
Upon reproduction, a polarized light beam having the intensity which is weaker than that of the recording light beam is made incident upon the opto-magnetic record medium. Then, the polarizing direction of the light beam reflected by the record medium is rotated in accordance with the direction of the magnetization (i.e., the recorded information) so that the original information can be reproduced by detecting the rotational direction of the polarization. It should be noted that the intensity of the external bias magnetic field is usually set in the range of 100 to 300 Oersteds.
In the known apparatus there is produced a large leakage of magnetic field toward the record medium. For instance, the intensity of the leaked magnetic field is about 50 to 150 Oersteds. Therefore, the recording magnetic field should be larger than the leakage of magnetic field, and thus it is necessary to make the bias magnetic field generating device 32 including the permanent magnet or coil large in size.
In order to avoid the above explained drawback, it has been proposed in Japanese Patent Application Laid-open Publication Kokai Sho 61-29429 to provide auxiliary yokes 33a and 33b as illustrated in FIG. 5. In this construction, the auxiliary yokes serve to close the magnetic circuit, so that the generation of the leakage of magnetic field can be avoided. However, in this known apparatus, it is necessary to provide the auxiliary yokes 33a and 33b separately from the yokes 23a and 23b, so that the number of parts is increased and the cost is increased and the manufacturing efficiency is decreased. Also, the weight of the pick-up device is increased.
In Japanese Utility Model Application Laid-open Publication Kokai Sho 61-21021, the whole magnetic circuit is covered with a protection cover 34 made of soft magnetic material as shown in FIG. 6 so as to decrease the influence of the leakage of magnetic field. In the known apparatuses, the size and weight of the whole pick-up device are increased by the provision of the auxiliary yokes 33a, 33b and protection cover 34, and thus the pick-up device cannot be moved stably at a high speed and therefore the high speed access is limited.